HSPA is generally based on High Speed Downlink Packet Access, HSDPA, in the downlink and Enhanced Uplink, EUL, in the uplink. The Enhanced Uplink is sometimes referred to as High Speed Uplink Packet Access, HSUPA.
HSDPA is an enhancement to WCDMA that provides a smooth evolutionary path to higher data rates. HSDPA includes additional transport and control channels such as the High-Speed Downlink Shared Channel, HS-DSCH. EUL includes additional transport and control channels such as the Enhanced Dedicated Channel, E-DCH.
HSDPA enables improvements in capacity and end-user perception by means of efficient sharing of common resources in the cell among many users, rapid adaptation of the transmission parameters to the instantaneous radio channel conditions, increased peak bit rates and reduced delays. Fast scheduling is a mechanism that selects which user(s) to transmit to in a given Transmission Time Interval, TTI. The packet scheduler is a key element in the design of a HSDPA system as it controls the allocation of the shared resources among the users and to a great extent determines the overall behavior of the system. In fact, the scheduler decides which users to serve and, in close cooperation with the link adaptation mechanism, which modulation, power and how many codes should be used for each user. This produces the actual end-users bit rate and system capacity. The High-Speed Downlink Shared Data Channel, HS-DSCH, is shared between users using channel-dependent scheduling to take advantage of favorable channel conditions in order to make best use of the available radio resources. The downlink control information is carried on the High-Speed Shared Control Channel, HS-SCCH.
Multiple Input Multiple Output, MIMO was introduced to increase peak data rates through multi-stream transmission. MIMO generally denotes the use of multiple antennas at both the transmitter and receiver. This can be used to obtain a diversity gain and thereby increase the carrier-to-interference ratio at the receiver. However, the term is also commonly used to denote transmission of multiple layers or multiple streams to improve the end-user throughput by acting as a ‘data-rate booster’ through spatial multiplexing. Naturally, improved end-user throughput will to some extent also result in an increased system throughput.
So-called dual-stream MIMO, also referred to as dual-branch MIMO, supports transmission of up to two streams or layers. Each stream is normally subject to the same physical-layer processing in terms of coding, spreading and modulation as the corresponding single-layer HSDPA case. Even if only a single stream is transmitted it can be beneficial to exploit both transmit antennas by using transmit diversity. To support dual-stream transmission, the HS-DSCH is modified to support up to two transport blocks per TTI. Each transport block represents one stream or layer. In effect, this means that up to two transport blocks may be simultaneously transmitted on the downlink shared data channel. The standardized HS-SCCH control channel is extended to include so-called rank information about the number of streams, i.e. the number of transport blocks to be simultaneously transmitted to the UE, one or two, and their respective modulation scheme as well as which pre-coding to be used. Reference can e.g. be made to the third generation partnership project, 3GPP, technical specification TS 25.212 V10.2.0
Current work within the third generation partnership project, 3GPP, regarding HSPA evolution include addition of several new features in order to meet the requirements set by the International Mobile Telecommunications Advanced, IMT-A. The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four-branch MIMO, i.e. utilize up to four transmit and receive antennas, to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. Four-branch MIMO, which is sometimes also referred to as four-stream or four-layer MIMO, provides up to 84 Mbps per 5 MHz carrier for high signal to noise ratio, SNR, users and improves the coverage for low SNR users. Four-branch MIMO supports simultaneous transmission of up to four streams or layers on the downlink to a given UE. The HS-DSCH is thus modified to support up to four transport blocks per TTI, where each transport block represents one stream or layer. In effect, this means that up to four transport blocks may be simultaneously transmitted on the downlink shared data channel.
Introduction of four-branch MIMO will however require a new control channel structure to send the downlink grant information to the UE. It would be desirable to provide a power-efficient solution for such a control channel in a HSPA system.